论上扬子区上奥陶统大渡河组

ON THE UPPER ORDOVICIAN DADUHE FORMATION OF THE UPPER YANGTZE REGION

在滇东北永善莲峰新发现的万和剖面,奥陶系-志留系交界地层发育良好、地层连续,化石丰富,含有笔石、腕足类、三叶虫、牙形类、几丁类和疑源类化石,包括了大渡河组、产Hirnantia动物群的观音桥层和龙马溪组。此处大渡河组可分为上、下两段,下段为灰岩与钙质泥页岩互层,上段为钙质泥岩夹灰岩。笔石自下而上可识别为Dicellograptus complexus带、Paraorthograptus pacificus带(下亚带、Tangyagraptus typicus亚带和Diceratograptus mirus亚带)以及Metabolograptus extraordinarius带,为凯迪晚期至赫南特早期。几丁类尚未在上段发现,下段属于Eisenackitina ripae带。上段底部(即Tangyagraptus typicus亚带底)产腕足类Kassinellasp.,Leptellinasp.,Paromalomenasp.等,属于深水BA5(底栖组合),在华南同期地层中未见及。观音桥层产典型的Hirnantia动物群,属于BA3,为标准的浅凉水组合。依据万和及相邻地区的资料,厘定了大渡河组。大渡河组分布于扬子台地西部(滇东北—川西南),与扬子台地内部广布的五峰组和扬子台地西南缘的铁足菲克组(大部分)、西缘的大箐组(上部)属同期异相。从产BA5腕足类的大渡河组上段底部,到产BA3赫南特贝动物群的观音桥层,再到龙马溪组下部黑色页岩,是奥陶纪末大灭绝第一幕和第二幕全球气候快速变冷和转暖、海平面迅速地先降后升在扬子海盆的充分反映。

The Daduhe Formation is a lithological unit in the Upper Ordovician rocks of western Yangtze Platform. It was established based on the section at Jiaodingshan of Hanyuan County, Sichuan Province. However, the lithology of the Daduhe Formation at the type locality is quite unique owing to its thin thick- ness, restricted distribution, and the presence of manganese ores. A newly found section at Wanhe of Yongshan County includes Upper Ordovician through Lower Silurian strata, cropping out along a country road near the Heping Village of Yongshan County, NE Yunnan, where the lithological succession across the Ordovician-Silurian boundary consists of, in an ascending order, the Daduhe Formation, the Kuany- inchiao Bed and the Lungmachi Formation. Lithologically, the Daduhe Formation can be subdivided into two members. The Upper Member is composed of calcareous mudstone, intercalated with argillaceous limestone and, the Lower Member consists of limestone interbedded with mudstone. Fossil groups of these strata include graptolites, brachiopods, trilobites, conodonts, chitinozoans and acritarehs. The graptolite succession of the Daduhe Formation is divided into, in an ascending order, the Dicellograptus complexus Biozone, the Paraorthograptus pacificus Biozone （including the lower subzone, the Tangyagraptus typi- cus Subbiozone, and the Diceratograptus rnirus Subbiozone）, and the Metabolograptus extraordinarius Biozone. Chitinozoans are assigned to the Eisenackitina ripae Biozone in the lower member. A braehiopod fauna, including Kassinella sp. , Leptellina sp. , Paromalomena sp. , etc. and referred to a depth of BA5, is obtained at the basal Upper Member and such a deep-water brachiopod fauna has not been discovered from the contemporary strata in other areas of South China. The Daduhe Formation is of late Katian age （Late Ordovieian） based on graptolites, brachiopods, and chitinozoans,corresponding to the span from the D. complexus Biozone to the basal M. extraordinarius Biozone. The Kuanyinchiao Bed bears the Hirnan- tia fauna, indicating a depth of BA3. The present paper emends the Daduhe Formation on the basis of the Wanhe section of Yongshan County and other sections in the western Yangtze Platform. Furthermore, the Wanhe section is proposed here to be an auxiliary section for the Daduhe Formation in consideration of its good exposure and prolific fossils. The emended Daduhe Formation is distributed in the western Yangtze Platform （NE Yunnan and SW Sichuan） and adjacent to the Kangdian Oldland. It can be correlated with the Wufeng Formation in the inner part of the Yangtze Platform, and much of the Tiezufeike Formation and the Daqing Formation （upper part） in the southwestern margin of the Yangtze Platform. The interval from the basal Upper Member of the Daduhe Formation （bearing brachiopods of BAS）, to the Kuany- inchiao Bed （yielding Hirnantia fauna of BA3）, and further upsection to the black shale of the Lungmaehi Formation, fully reflects the rapid global cooling （the first pulse） and then warming （the second pulse）, and an eustatic sea-level falling and then rising in the Yangtze Sea during the end-Ordovician mass extinc- tion.